1. Field of the Invention
The present invention relates to a webbing retractor for tensioning a webbing belt in a direction in which a vehicle occupant is restrained when a vehicle speed is rapidly decreased, and more particularly, to a webbing retractor comprising a so-called force limiter mechanism for allowing the webbing belt to be pulled out by a predetermined amount and which absorbs energy when unwinding of the webbing belt is prevented.
2. Description of the Related Art
A webbing retractor forming a seat belt device of a vehicle is usually provided with a locking mechanism for locking a spool (a winding shaft) to prevent unwinding of a webbing belt when a vehicle speed is rapidly decreased.
An example of the locking mechanism comprises a lock base which is disposed coaxially with a spool and integrally coupled with the spool; and a locking means which engages with the lock base to prevent rotation of the lock base when a vehicle speed is rapidly decreased and thus indirectly prevents rotation of the spool.
In the webbing retractor such as described above, a so-called force limiter mechanism is provided for allowing a webbing belt to be pulled out by a predetermined amount to effect energy absorption when unwinding of the webbing belt is prevented. In an example of this force limiter mechanism, one end portion of a torsion bar disposed coaxially with the spool is fixed to the spool, and the other end portion thereof is integrally coupled with the above-described lock base. That is, in this type of force limiter mechanism, the spool is made to rotate relative to the lock base by locking the lock base with the locking means when the vehicle speed is rapidly decreased as well as by a tension applied to the webbing belt from the body of the vehicle occupant moving toward a front side of the vehicle when the vehicle speed is rapidly decreased. A rotating force of this relative rotation deforms the torsion bar in a direction in which the torsion bar is twisted, thereby absorbing energy and allowing rotation of the spool by the predetermined amount.
The above-described absorbed energy corresponds to a load (a force limiter load) applied to the webbing belt which increases in proportion to the unwound amount of the webbing belt (an amount of rotation of the spool) from when the spool begins to rotate up until the point where the spool has rotated by the predetermined amount. Therefore, the force limiter load is small when the spool begins to rotate. However, from a viewpoint of initial restraining performance when the vehicle speed is rapidly decreased, it is preferable to obtain a large force limiter load as soon as possible.
In view of the aforementioned, an object of the present invention is to provide a webbing retractor comprising a force limiter mechanism which can improve initial restraining performance.
In order to solve the aforementioned problems, according to the present invention, there is provided a webbing retractor comprising:
(a) a webbing belt;
(b) a rotatable spool for winding the webbing belt;
(c) a locking element provided at one axial-direction side of the spool for locking rotation of the spool in a webbing pay-off direction; and
(d) an energy absorbing structure comprising a torsion bar and a shear element and provided between the spool and the locking element for coupling the spool and the locking element so as to allow rotation of the spool in the pay-off direction under heavy strain.
In accordance with another aspect of the present invention, there is provided a retractor for a vehicle safety belt comprising:
(a) a housing;
(b) a ratchet provided at the housing;
(c) a spool rotatably attached in the housing for winding a webbing;
(d) a lock base provided at one axial-direction side of the spool coaxially with the spool and comprising a locking member which is engageable with the ratchet so as to lock rotation of the lock base; and
(e) an energy absorbing structure provided between the spool and the lock base for coupling the spool and the lock base so as to allow rotation of the spool in the webbing pay-off direction under heavy strain;
wherein, the energy absorbing structure comprises (i) a torsion bar having opposite ends, one of the opposite ends being fixed integrally at the spool, the other of the opposite ends being fixed integrally at the lock base; and (ii) a plurality of shear ribs disposed between the spool and the lock base and spaced apart from each other.